Final answer:
The rate of diffusion across a selectively permeable membrane will be slow when temperatures are low (II only), as reduced temperatures slow molecular movement and consequently the diffusion process.
Step-by-step explanation:
The rate of diffusion across a selectively permeable membrane will be slower when certain conditions are met. Specifically, if the temperatures are low (II only), the molecular movement slows down, hindering the diffusion process. Contrary to what might seem intuitive, steep concentration gradients and small solute molecules actually contribute to a faster rate of diffusion. Steep concentration gradients (I) provide a larger difference in concentration between the two sides of a membrane, thus promoting more rapid diffusion until dynamic equilibrium is reached. Smaller molecules (III) diffuse more quickly due to the ease with which they can navigate between the molecules of the substance they are moving through. Therefore, the correct answer to the question is that the rate of diffusion will be slow when II only is true - temperatures are low.